An optoelectronic device, such as a laser diode or a photo diode, is generally enclosed in a transistor outline (TO) package, which provides a conductive housing for the optoelectronic device. A laser diode converts an electrical signal into an optical signal for transmission over a fiber optic cable, while a photo diode converts an optical signal into an electrical signal. In order for a laser diode to convert an electrical signal into an optical signal, the electrical signal must be sent through the TO package of the laser diode. Similarly, an electrical signal from a photo diode must be sent through the TO package of the photo diode to external electrical circuitry. For high frequency operation, it is important to control the impedance seen by the electrical signals that flow into and out of the TO package.
Conventional external electrical circuitry include circuit boards with one or two ground planes for each signal trace. In these conventional packages, the ground planes are equidistant from a given signal trace. But, because the distance between the signal traces and the one (or two) ground planes does not vary, impedance adjustments for a given signal trace are typically made by varying the width of the signal trace instead. However, at high frequencies (e.g., approximately 10 GHz), varying the widths of signal traces degrades data signal integrity. In other words, a variation in the width of a signal trace creates a signal impedance discontinuity, which causes signal reflections. Persons skilled in the art recognize that the impedance of a signal trace can be thought of as the “rate of flow” of the signal trace. When the electrons of a signal from a higher impedance signal trace attempt to travel through a lower impedance signal trace, some of the electrons cannot make it and are reflected back through the higher impedance signal trace.